1. Field of the Invention
The present invention relates to a magnetic recording medium having at least one magnetic layer provided over a non-magnetic support.
2. Description of the Related Art
As tape-form magnetic recording media for audio, video, and computers, and disc-form magnetic recording media such as flexible discs, a magnetic recording medium has been used in which a magnetic layer having dispersed in a binder a ferromagnetic powder such as γ-iron oxide, Co-containing iron oxide, chromium oxide, or a ferromagnetic metal powder is provided on a support. With regard to the support used in the magnetic recording medium, polyethylene terephthalate, polyethylene naphthalate, etc. are generally used. Since these supports are drawn and are highly crystallized, their mechanical strength is high and their solvent resistance is excellent.
Since the magnetic layer, which is obtained by coating the support with a coating solution having the ferromagnetic powder dispersed in the binder, has a high degree of packing of the ferromagnetic powder, low elongation at break and is brittle, it is easily destroyed by the application of mechanical force and might peel off from the support. In order to prevent this, an undercoat layer is provided on the support so as to make the magnetic layer adhere strongly to the support.
On the other hand, magnetic recording media are known in which a radiation-cured layer is formed using a compound having a functional group that is cured by radiation such as an electron beam, that is, a radiation curing compound (ref. JP-B-5-57647, JP-A-60-133529, JP-A-60-133530, and JP-A-60-133531; JP-B denotes a Japanese examined patent application publication, and JP-A denotes a Japanese unexamined patent application publication). These radiation-cured layers formed from the radiation curing compound have poor adhesion to the magnetic layer, and when such a magnetic recording medium, for example, a video tape, is run repeatedly in a VTR, a part of the magnetic layer peels off, thus giving rise to the problem of faults such as dropouts. Furthermore, when coating the radiation-cured layer with a magnetic layer, since the surface of the radiation-cured layer is smooth it easily sticks to web handling rollers, and this might cause faults due to the occurrence of creases.
Recently, a playback head employing MR (magnetoresistance) as the operating principle has been proposed, its use in hard disks, etc. has started, and its application to magnetic tape has been proposed. The MR head gives a playback output several times that of an induction type magnetic head; since it does not use an induction coil, equipment noise such as impedance noise is greatly reduced, and by reducing the noise of the magnetic recording medium it becomes possible to obtain a large S/N ratio. In other words, by reducing the magnetic recording medium noise, which had previously been hidden by equipment noise, recording and playback can be carried out well, and the high density recording characteristics are outstandingly improved.
However, the MR head has the problem that it generates noise (thermal noise) under the influence of microscopic heating; in particular, it has the problem that when it hits a projection present on the surface of a magnetic layer, the noise suddenly increases and continues, and in the case of digital recording the problem can be so serious that error correction is impossible. This problem of thermal noise becomes serious in a magnetic recording medium used in a system in which a recorded signal having a recording density of 0.5 Gbit/inch2 or higher is replayed.
In order to reduce such thermal noise, it is important to control the surface properties of the magnetic layer, and there has been a desire for suitable means to do this.
In order to improve the smoothness and the transport durability of the magnetic recording medium, a magnetic recording medium has therefore been proposed that contains, in a magnetic layer or a middle layer, a radiation-curing polyfunctional curing agent and, as a binder, a polyurethane in which a magnetic powder and a non-magnetic powder have high dispersibility (ref. JP-A-2002-133637). However, it is necessary for the surface of a support that is used for this magnetic recording medium to be extremely smooth, and the friction of the support during a coating step is therefore high and the support sticks, thus readily causing faults due to the occurrence of creases and thereby degrading the productivity.